Downhole rotary pumps, such as progressing cavity pumps, are used for the conveying of different types of fluids, but are especially well suited for the pumping of very viscous or thick liquids such as crude oil laden with sand. A downhole rotary pump is operated by a drive string, generally consisting of a rod or tube string that extends down the well bore. The drive string rotates in a stationary production tubing and is suspended from and rotated by a drivehead assembly which is associated with the wellhead. The drivehead assembly is generally mounted to the top of a wellhead which is attached to the top end of the production tubing. A polished rod is attached to the upper end of the drive string and extends through the drivehead to facilitate sealing around the drive string. The pump includes a stator connected to the bottom end of the production tubing and a rotor attached to the bottom end of the drive string. Upon actuation of the pump by rotation of the drive string, the pumped liquid is forced to the ground surface through the annular space provided between the string and the production tubing.
In conventional downhole rotary pump driveheads, a stuffing box is used to seal the annular space between the tubing and the string in the drivehead and above the pumped liquid take-off valve of the wellhead. The stuffing box is generally mounted in a bottom end of the drivehead to tightly seal around the rotating polished rod. In order to reduce wear of the stuffing box and to provide adequate lubrication of the polished rod/stuffing box interface, the fit of the stuffing box around the string is adjusted such that a controlled leakage of about 2 to 3 drops of pumped liquid per minute is achieved. This constant leakage, although at a relatively low rate, can result in the accumulation of significant amounts of spilled liquid over long periods of operation. Recently, political pressure by environmental groups and the enactment of laws in many jurisdictions which require "clean" wellheads, have forced well operators to reduce the stuffing box leakage rate. However, the resulting lesser lubrication causes considerably higher stuffing box and polished rod wear. Furthermore, wobble of the polished rod, which is especially prevalent with Moineau type pumps and must be compensated by the stuffing box if a reliable seal is to be achieved, places additional stress on the sealing material respectively used. Thus, a reliable and durable seal is desired, which will not allow unacceptable leakage.